1. Field of the Invention
The present invention relates to a method of assigning a frequency from a base station to mobile stations in a CDMA (Code Division Multiple Access) communication system.
2. Description of the Related Art
The CDMA communication system is expected to be the next-generation mobile communication system. The CDMA communication system is advantageous in that a plurality of adjacent base stations can share one frequency band.
In a CDMA communication system, different codes are assigned to respective channels, and signals are demodulated on the basis of the fact that there is a difference between autocorrelated values, which are correlated values between the same codes, and cross-correlated values, which are correlated values between different codes. It is therefore necessary that the cross-correlated values between the codes assigned to the respective channels are sufficiently lower than the autocorrelated values. It is most preferable that all the codes which are used be orthogonal to each other (the cross-correlated values be 0). However, since it is difficult for all the codes which are used by base stations that are adjacent to each other to be orthogonal to each other, the cross-correlated values are generally not 0.
If the cross-correlated values are not 0, then there is a possibility that when a mobile station A1 is present near a base station and a mobile station A2 is present far from the base station, depending on the ratio of transmission powers of the mobile stations A1, A2 at the time signals from the mobile stations A1, A2 arrive at the antenna of the base station, the correlated value (cross-corrected value) between the signal from the mobile station A1 and the code assigned to the mobile station A2 may be greater than the correlated value (autocorrelated value) between the signal from the mobile station A2 and the code assigned to the mobile station A2.
In such a case, because the cross-corrected value is not lower than the autocorrelated value, the signal from the mobile station A2 cannot properly be demodulated. This could lead to a fatally dangerous situation where if the signal power sent from one mobile station and received by the antenna of a base station is much greater than the signal power sent from another mobile station and received by the antenna of the base station, then the base station is unable to decode signals other than the signal from the mobile station whose reception power is larger at the antenna of the base station. This problem is widely known as a near-far problem. In order to solve the problem, it is necessary to perform a transmission power control process on transmitters for equalizing signal powers from all the transmitters in input signals received by a receiver.
If a mobile communication system is operated by a sole network operator and a handoff is properly carried out as a mobile station moves, then communications are effected via a path whose transmission loss is smallest between the base station and the mobile station. It is thus possible to perform a transmission power control process in order to minimize the transmission power of the mobile station for thereby solving the near-far problem.
Another problem occurs when a plurality of mobile communications are operated in one area by a plurality of network operators. In this situation, if the base stations are located at one site or mutual roaming is carried out between the different network operators, then no difficulty arises because a path whose transmission loss is smallest between base and mobile stations is practically selected. Since, however, such a solution is generally difficult to achieve, it has been customary to solve the above problem by assigning different frequency bands-to the respective network operators, as shown in FIG. 1 of the accompanying drawings. FIG. 1 shows the assignment of four frequency channels to each of network operators A, B, C.
If the frequency bands assigned to the respective network operators are successive, then in frequency channels located at a boundary between different frequency bands, the leaking power in an adjacent channel of a transmission signal from a mobile station causes a near-far problem to a frequency channel of another network operator. For example, in frequency channels fa1, fa4 located at a boundary of the frequency band assigned to a network operator A, the leaking power in an adjacent channel of a transmission signal from a mobile station causes a near-far problem to a frequency channel fc4 of a network operator C and a frequency channel fb1 of a network operator B.
FIG. 2 of the accompanying drawings shows CDMA communication systems provided by a plurality of network operators and located in one area. As shown in FIG. 2, two network operators A, B provide respective CDMA communication systems in one area. The CDMA communication system provided by the network operator A has base stations (not shown) which communicate with mobile stations present in cells A1-A7. It is assumed that the cells A1-A7 overlap a cell B1 of a base station of the CDMA communication system provided by the network operator B, and that the frequency channels shown in FIG. 1 are assigned to each of the network operators A, B.
In FIG. 2, xcex1 represents the position of the base station of the cell A1, xcex2 represents the position of the base station of the cell B1, and xcex3 represents the position of the base station of the cell A4.
In order to solve a near-far problem in one CDMA communication system, each of the base stations control the transmission powers of mobile stations linked thereto, and the reception powers at the antenna of each of the base stations are controlled at a constant level at all times irrespective of the positions of the mobile stations. Specifically, if the distance between a mobile station and a base station linked thereto is large, then the transmission power of the mobile station is increased, and if the distance between a mobile station and a base station linked thereto is small, then the transmission power of the mobile station is reduced.
In FIG. 2, when a mobile station belonging to the CDMA communication system provided by the network operator A moves from the position xcex1 to the position xcex2 to the position xcex3, the transmission power of the mobile station is first progressively increased from a low level to a maximum level at the position xcex2. Near the position xcex2, a handoff takes place from the base station of the cell A1 to the base station of the cell A4. As the mobile station approaches the position xcex3, the transmission power of the mobile station is gradually lowered.
The transmission power of a mobile station in the cell A1 which uses the frequency channel fa4 changes as shown in FIG. 3a of the accompanying drawings. The reception power at the base station of the cell A1 in the position a and the reception power at the base station of the cell A4 in the position xcex3 change as shown in FIG. 3b of the accompanying drawings.
As shown in FIG. 3b, when the mobile station is located between the position xcex1 and the position xcex2, the reception power at the base station of the cell A1 is constant, and when the mobile station is located between the position xcex2 and the position xcex3, the reception power at the base station of the cell A1 is gradually reduced. The reception power at the base station of the cell A4 is gradually increased when the mobile station is located between the position xcex1 and the position xcex2, and is constant when the mobile station is located between the position xcex2 and the position xcex3. When the reception powers are constant, the reception powers have the same level at the base stations of the cells A1, A4. At this time, the reception power in the frequency channel fb1 of the base station in the position xcex2 of the cell B1 of the network operator B is indicated by the solid-line curve in FIG. 3c of the accompanying drawings because of the leaking power in an adjacent channel from a mobile station. The conventional CDMA communication system has strict standards for the leaking power in an adjacent channel with respect to a transmitted signal from a mobile station, so that any interference which occurs with the base station of the cell when the mobile station is in the position xcex2 can be reduced to a level low enough not to cause practical problems to the CDMA communication system.
The reception power in the frequency channel fb1 in the base station of the cell B1 at the time the mobile station has a transmission frequency channel fa3 is indicated by the dotted-line curve in FIG. 3c. The reception power in the frequency channel fb1 is low because the leaking power is exponentially reduced as the reception frequency channel is away from the transmission frequency channel.
As described above, the conventional CDMA communication system has strict standards for the leaking power in an adjacent channel with respect to a transmitted signal from a mobile station so as to solve the near far problem. Therefore, the transmitters of the mobile stations, particularly the final-stage power amplifiers thereof, are required to have a a high level of linearity, and hence have a large consumption power requirement and an increase in size.
It is an object of the present invention to provide a method of assigning a frequency from a base station to mobile stations in a CDMA communication system for solving a nearfar problem without strict. standards for the leaking power in an adjacent channel with respect to the transmitter of a mobile station and for reducing a consumption power requirement and size of base stations.
To achieve the above object, in a method of assigning a frequency in a CDMA communication system according to the present invention, a base station measures a propagation loss over a channel between the base station and a mobile station. If the measured propagation loss is greater than a certain fixed value, then the base station determines that since the mobile station is present in a position distant from the base station, the mobile station may possibly interfere with a base station of another CDMA communication system. The base station assigns a frequency channel which is not adjacent to the frequency band assigned to the other CDMA communication system, to the mobile station which may possibly interfere with a base station of another CDMA communication system. Therefore, it is possible to reduce interference with a base station belonging to another network operator, without strict standards for the leaking power in an adjacent channel of a transmitted signal of a mobile station. Therefore, the consumption power requirement and size of the transmitter of the mobile station can be reduced.
In another method of assigning a frequency in a CDMA communication system according to the present invention, a frequency channel which is not adjacent to the frequency band assigned to another CDMA communication system is assigned to a mobile station whose transmission power level is greater than a certain fixed value.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.